This invention relates to a method of and apparatus for producing nitrogen by the separation of air.
The separation of air by rectification is very well known indeed. Rectification is a method in which mass exchange is effected between a descending stream of liquid and an ascending stream of vapor such that the ascending stream of vapor is enriched in a more volatile component of the mixture to be separated and the descending stream of liquid is enriched in a less volatile component of mixture to be separated.
Conventionally, air is separated in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler, of which the condensing passages communicate with an upper region of the higher pressure rectification column and the reboiling passages communicate with a lower region of the lower pressure rectification column. Nitrogen is thereby separated in the higher pressure rectification column and is condensed in the condenser-reboiler. Part of the resulting condensate is used as reflux in the higher pressure column and another part of the condensate is so used in the lower pressure rectification column. An oxygen-enriched liquid air fraction is taken from the bottom of the higher pressure rectification column and is introduced into an intermediate mass exchange region of the lower pressure rectification column. A nitrogen fraction is obtained at the top of the lower pressure rectification column and an oxygen-enriched fraction at its bottom. A nitrogen product is therefore obtained at the pressure of the lower pressure rectification column. Many industrial processes, for example, the enhanced recovery of oil or gas, require nitrogen to be supplied at an elevated pressure, often well in excess of that at which the higher pressure rectification column operates. In order to reduce the amount of work required to raise the pressure of the nitrogen product from that of the lower pressure rectification column to that demanded by the process to which the nitrogen is to be supplied, it is known to take some of the nitrogen product as vapor from the higher pressure rectification column. A feature of such processes is that for a given size of air separation plant and a given purity and pressure of the nitrogen product, the total power consumption at first falls with increasing nitrogen recovery to a minimum and then rises again. This phenomenon results from two opposing factors. The ideal separation work (and hence power consumption) is at a minimum when the nitrogen recovery is very low and the waste product is still essentially air. It is at a maximum when the waste gas contains no nitrogen. However, the process efficiency (actual work input/ideal work input) is very low when the recovery is very low because the plant is much bigger than it needs to be and losses of work arising from pressure drops and temperature differences are large. Conversely, when the recovery is high, the process efficiency is higher. As the recovery is reduced from 100%, there is a minimum power at an optimum recovery, where the falling separation power is just balanced by the increasing losses of work that are caused by the plant getting larger. The total power consumption of the process also typically includes the power consumed in compressing the nitrogen product. Taking a part of the nitrogen product from the higher pressure column reduces the power consumed in compressing the nitrogen products but reduces the nitrogen recovery.
Other expedients may also decrease the nitrogen recovery. For example, the production of a liquid nitrogen product requires a part of the incoming air to be condensed. This in turn reduces the vapor flow available for condensation in the condenser-reboiler. Again, in order to compensate a larger, less efficient, plant is required.
In practice, known double column air separation plants for generating nitrogen are not necessarily designed either for a minimum power consumption or for maximum nitrogen recovery. Rather, there is generally a preferred operational envelope represented by a particular region of a graph of power consumption plotted against nitrogen recovery, the actual optimum depending on extraneous economic circumstances. It is aim of the present invention to provide methods and apparatuses for producing nitrogen which effectively enable the preferred operational envelope to be shifted in the direction of reduced power consumption without reducing nitrogen recovery, or in the direction of increased nitrogen recovery without increasing power consumption, or in both directions.